Axially symmetric circulations in a moist atmosphere

The role of the planetary boundary layer in the cross-equatorial Hadley circulation is investigated. An axially symmetric atmosphere presents a Hadley-like overturning circulation whenever the horizontal gradient of radiative-convective equilibrium temperature is above a given threshold. The upper branch of such a circulation transports mass from the regions of strong precipitation to the subtropics. Because of conservation of angular momentum, the return flow in the lower troposphere can only occur in two ways: (1) within the boundary layer, where surface friction and turbulence allow air masses to change angular momentum, or (2) within the free troposphere, in which case the flow must either follow surfaces of constant angular momentum or occur within a region of homogenized angular momentum.

The meridional mass transport in the planetary boundary layer depends on the zonal wind above the boundary layer, on the boundary-layer thickness, on the surface drag coefficient, and on the Coriolis parameter. In the case of cross-equatorial Hadley circulations, an upper bound on the cross-equatorial mass transport within the boundary layer can be derived. As a result, most of the return flow crosses the Equator in the free troposphere. In addition, there is a region of low level convergence and increased precipitation on the winter side of the Hadley circulation. These findings are confirmed by numerical simulations with an idealized axially symmetric model.